1. Field of the Invention
The present invention relates to an optical disc drive and its control method and in particular, to a servo control method and an optical disc drive appropriate for implementing the control method.
2. Description of the Related Art
A conventional optical disc drive usually uses a servo signal processor circuit (hereinafter, referred to as an SSP) as hardware for carrying out a servo processing. An error signal outputted from an optical unit is supplied to the SSP so as to be compared to a target value and a difference between the error signal and the target value is corrected for carrying out the servo processing. FIG. 8A is a block diagram showing configuration example of an optical disc drive for carrying out such a servo processing with hardware.
Referring to FIG. 8A, according to a reproduction signal read out from an optical disc 1 by an optical unit 2, an RF circuit 4 creates a servo signal and an SSP 6 carries out a servo processing via a driver circuit 3 such as a focus servo, tracking servo, sled servo, and spindle servo. A microcomputer 5 carries out a system control such as a servo control sequence, disc information management, key-in information fetch, display control, and other system control.
The SSP 6 for the servo processing is constituted by hardware for carrying out the servo processing in parallel processing method. In this case, a filter calculation for determining a control amount for correcting the aforementioned difference between an error signal and a target value is carried out by combining an fs process with an 1/n fs process. The fs process is carried out for each sampling frequency (hereinafter, referred to as fs), and the 1/n fs process is carried out 1/n times of the sampling frequency. That is, a focus servo and a track servo need be controlled at a high speed, whereas a spindle servo need not be controlled at such a high speed as the focus and tracking servo. Accordingly, as shown in FIG. 9, the calculation amount varies depending on the sampling timing as follows: only an fs processing, or an fs processing in combination with a 1/4 fs processing, or an fs processing in combination with a 1/4 fs processing and 1/8 fs processing, and the like.
It should be noted that for carrying out a servo processing through hardware, there is also an optical disc drive using the SSP6 and the microcomputer 5 which are made as a unitary block. In this case also, the system control of the servo control is carried out by the microcomputer portion and the servo processing is carried out by hardware.
In the optical disc drive shown in block diagrams of FIG. 8A and FIG. 8B, the SSP 6 is hardware for carrying out a servo processing in a parallel processing method. However, it is desired to perform the servo processing with microcomputer software. If the servo processing is carried out by software, it becomes easy to modify the filter form such as the calculation order and calculation amount for general purpose. For example, a CD has been the main optical disc widely used, but recently new discs such as a PD (private disc, writable CD) and DVD (digital versatile disc) have been developed and are now in use. A servo processing through software would enable a single servo LSI to be used for various types of media.
However, if the servo processing which has been conventionally carried out by hardware is simply transferred to a microcomputer software, it is necessary to employ a microcomputer having a considerably high speed. This is because the processes carried out in parallel in the conventional hardware need be carried out in series in software, requiring a long calculation time at once. Moreover, as shown in FIG. 9, down sampling processes are concentrated in a particular sampling timing, and the time required for calculation greatly varies depending on the sampling timing.